1. Field of the Invention
The present invention relates to a control method for maintaining point of common coupling (PCC) voltage of a wind farm, and more specifically, to a method of promptly recovering PCC voltage by supplying each wind turbine with a portion of reactive power needed for recovering the voltage in proportion to available reactive power when a disturbance such as voltage reduction occurs at a bus close to the point of common coupling.
2. Background of the Related Art
In order to continue stable operation of a power grid, bus voltages of a system should be maintained within an allowed range when a relatively small disturbance such as a load change occurs in the system. In addition, when a relatively big disturbance such as a short circuit fault of a power grid occurs, voltages of the system bus should be promptly recovered within the allowed range after the fault is removed, as well as while the fault lasts. When a voltage of the grid bus goes out of the allowed range by the disturbance, the voltage of the corresponding bus is maintained within the allowed range through reactive power compensation provided by a reactive power compensation device or a synchronous generator located in the neighborhood of the corresponding bus. In the case of a power grid having the high wind penetration, wind turbines in the wind farm should have a capability of supplying reactive power to a point of common coupling to recover voltage at a point connected to the power gird to a rated voltage when a disturbance which invites voltage reduction occurs in the power grid.
Currently, since a variable speed wind turbine mainly used for generation of wind power is provided with a converter, it can be used as a reactive power generation source which supplies or consumes reactive power depending on the circumstances of the power grid. A doubly-fed induction generator is provided with a converter. Referring to FIG. 1, the doubly-fed induction generator is provided with a Rotor Side Converter (RSC) and a Grid Side Converter (GSC).
The Rotor Side Converter (RSC) may control active power and reactive power of a stator winding, perform a maximum power point tracking (MPPT) control to maximize active power of the stator, and perform a voltage control function for maintaining a rated voltage at a stator terminal or injecting reactive power into the power grid. On the other, the Grid Side Converter (GSC) is used to control a DC link voltage and may inject reactive power into the power grid when a disturbance occurs in the power grid. In addition, the doubly-fed induction generator may be further provided with a crowbar for short-circuiting rotor windings through a resistor in order to protect converters of the doubly-fed induction generator from over-current generated when a fault occurs in the power grid.
FIGS. 2A and 2B are views schematically showing a voltage control method according to a first convention technique. Referring to FIG. 2A, a wind farm (WPP) controller converts a difference between a voltage upcc measured at the point of common coupling (PCC) and a reference voltage uref of the point of common coupling, i.e., a voltage error (Δu), into a reactive power compensation value Qref using a Proportional Integral (PI) controller and then calculates a reactive power set value QWGiref multiplied by a different weighting factor
  (            P      avg              P      WGi        )for each wind turbine in the wind farm through the mathematical expression shown below.
      Q    ref    WGi    =                    P        avg                    P        WGi              ×          Q      ref      
Here, i denotes a sequence number of each wind turbine in the wind farm, Pavg denotes an average active power for all wind turbines in the wind farm, and PWGi denotes an active power output of the i-th wind turbine. Referring to FIG. 2, a wind turbine (WG) controller finally outputs a reactive current Idr—ref based on the reactive power set value QWGiref received from the WPP controller. However, when the voltage control method of the first conventional technique according to FIG. 2 is utilized, although it is advantageous in that voltage upcc of the point of common coupling (PCC) can be promptly recovered to the reference voltage uref, there is a problem in that a big overshoot is unavoidable since excessive reactive power more than needed is injected into the power grid when a high weighting factor is multiplied.
FIG. 3 is a view showing an example of a relation between a voltage error Δu and a reactive power compensation value Qref according to a grid code, and FIG. 4 is a view schematically showing a voltage control method according to a second conventional technique.
Referring to FIG. 3, it can be confirmed that according to the grid code regulating to have a voltage error tolerance of ±5% with respect to the reference voltage value uref of the point of common coupling and a power factor of 0.95, a control slope having a predetermined slope kQ of 6.6 is formed by the difference between the ‘maximum reactive power of approximately +0.33pu’ and the ‘minimum reactive power of approximately −0.33pu’ in the section of (−0.05pu)≦Δu≦(+0.05pu) of the voltage error Δu with respect to the reference voltage value uref of the point of common coupling.
Referring to FIG. 4A, a wind farm controller calculates a reactive power compensation value Qref by multiplying the difference between the actual measurement voltage upcc of the point of common coupling (PCC) and the reference voltage value uref of the point of common coupling, i.e., a voltage error Δu, and the slope kQ of the control slope. A reactive power error value QPCC is calculated by subtracting a reactive power measured at the point of common coupling from the reactive power compensation value, and a compensation reference voltage error value is calculated by proportionally integrating the reactive power error value.
Referring to FIG. 4B, a wind turbine controller calculates a voltage error value by subtracting a voltage value of the output terminal of a wind turbine from a sum of a reference voltage value of the wind turbine and the compensation reference voltage value and calculates a reactive current compensation value Idr—ref by multiplying the voltage error value and a conversion gain kQi. The conversion gain kQi multiplied in the process of calculating the reactive current compensation value from the voltage error value may be, for example, 2 and is equally applied to other wind turbines.
However, when the voltage of a point of common coupling is controlled according to a second conventional technique, since a reactive current compensation value Idr—ref is calculated by equally multiplying only the same slope kQi as shown in FIG. 4 without considering the active energy of each wind turbine changing by the wind speed or the like with time, there is a limit in that available reactive power changing together with the active power changing according to the wind speed in each wind turbine cannot be sufficiently utilized, and, as a result, there is a problem in that generation of an error in the neighborhood of the reference voltage uref of the point of common coupling is unavoidable in a steady state after a disturbance.